I have found if I go too long Find my iPhone will stop working. Once it stops the only way I can make it work is to reboot it. Ever since I noticed that I started to try to remember to reboot it once a week or so. Probably just an isolated problem, iPhone 4S Verizon iOS 7.1.2. But even if I bring up Find my iPhone on the phone, no other device or computer can find it, its strange.